The invention is based on a priority application EP 01 440 412.3 which is hereby incorporated by reference.
The present invention relates to a diplexer, particularly for use in microwave devices, comprising a low-pass filter and a high-pass filter, both having inductors and capacitors.
Generally, a diplexer is a three-port frequency-dependent device that may be used for separating or combining signals. The diplexer comprises a low-pass filter for extracting signals in a low frequency band and a high-pass filter for extracting signals in a high frequency band. Both, the low-pass filter and the high-pass filter are designed as passive units using capacitors and inductors for realising the mentioned functions.
A diplexer as mentioned above is for example known from U.S. Pat. No. 5,793,265. This diplexer also comprises a high-pass filter and a low-pass filter and serves the function of separating an incoming television signal from an incoming telephony signal, so that each of these signals can be processed by separate receivers when in a private home. Each of the filters is constructed in the form of a ladder network having series branches and parallel branches. In the high-pass filter, the series branches comprise capacitors and the parallel branches comprise inductors, with one of the parallel branches having a series inductor-capacitor circuit. In the low-pass filter, the series branches comprise inductors and the parallel branches comprise capacitors, one of the parallel branches having an inductor-capacitor series circuit.
In this document, it is proposed that all of the inductors of both filters be constructed as toroids. However, the problem is that these passive components have a physical size which makes it difficult to further minimize the diplexer. Moreover, the inductors have a bad quality factor in a frequency range above 1 GHz.
It has also turned out that using active filters instead of the passive filters shown for example in the above-mentioned prior art document is not an appropriate approach to the mentioned problem due to the relatively poor characteristics of such active filters. These characteristics make them insufficient for use in diplexers.
In view of the above, it is the object of the present invention to provide for a diplexer as mentioned in the outset which overcomes the problems mentioned above. Particularly, it is an object of the present invention to provide for a diplexer having a compact design, a high quality factor and stability, and being less expensive than prior art diplexers.
This object is solved by a diplexer particularly for use in microwave devices, comprising a low-pass filter and a high-pass filter, both having inductors and capacitors, wherein at least one inductor of each of said filters is provided as an active inductor.
That means in other words that spiral inductors, like toroids, are replaced with active components being constituted by capacitors, resistors and transistors and having the electrical characteristic of an inductor. The advantage of the diplexer according to the present invention is a substantial reduction of size since an active inductor builds smaller than a corresponding spiral inductor. Preferably, a reduction of about 90% may be achieved.
A further advantage of the diplexer according to the present invention is a cost reduction of the diplexer due to smaller packaging. Hence, the diplexer according to the present invention is less expensive compared to prior art diplexers.
A further advantage of the diplexer according to the present invention is an approvement of the quality factor and the stability.
The diplexer according to the present invention may be designed using GaAs technology. However, it is preferred to implement the diplexer using SiGe technology since the consumption of the diplexer is lower.
In a preferred embodiment, said active inductor having an input and an output terminal, said terminals being connected by a first branch comprising a first resistor, a first capacitor, a drain-source path of a first transistor and a second capacitor; a second branch comprising a gate-source path of a second transistor, a source-gate path of a third transistor, a third capacitor, a gate-source path of said first transistor and said second capacitor; a third branch comprising a gate-drain path of said second transistor, a fourth capacitor and a second resistor; a fourth branch comprising said gate-source path of said second transistor, a source-gate path of said third transistor, a fifth capacitor and said second resistor; and a fifth branch comprising said gate-source path of said second transistor, said source-drain path of said third transistor, a sixth capacitor and said second resistor.
This design has turned out as advantageous in view of its electrical characteristics. However, it is to be understood that other designs may be possible and the present invention is not limited thereto.
In a further embodiment, the low-pass filter comprises series and parallel branches, the series branches having inductor elements and active inductors alternately arranged, and the parallel branches having inductor elements and capacitor elements serially connected. Preferably, said high-pass filter comprises series and parallel branches, the series branches having capacitor elements, and the parallel branches having either inductor elements or active inductors, and capacitor elements serially connected.
It is to be understood that other inductor elements of the low-pass and high-pass filters may be replaced with active inductors.
Further features and advantages can be taken from the following description and the enclosed drawings.
It is to be understood that the features mentioned above and those yet to be explained below can be used not only in the respective combinations indicated, but also in other combinations or in isolation, without leaving the scope of the present invention.